OTWO has a great suggestion. When I did my fade, I deleted EVERY JW off of Facebook...and anyone I thought was a "mutual friend" with JW's that I knew got the ax, too, as well as one person who my ex knew I was friends with and stalked her page to get info and photos of me to send to the elders...he tried, but failed to get me DF'ed lol. I am 100% myself on Facebook because mine is set to completely private. Another of my exJW friends that I met on here has a Facebook page that is an alias so he can be himself as well. He has his regular page for JW friends and family, and then his alias page where he is free. So, both very good suggestions that have worked for some of us!
As for your sister, this conflict is between you and her and you need to take your mother out of the equation. Stop using her as the go-between. You and your sis are both adults. If your mom tries to insist on giving you messages from your sister or tries to tell you that your sister is upset about something you said or did, kindly tell your mom that if your sister has an issue, she needs to follow Jesus' advice and come to you about it first. That's the Christian thing to do, is it not? Involving others is not the way to resolve conflict. Your mother isn't an impartial mediator, she is your mother...and her feelings will bias her.
Now, knowing how you feel about the organization, your sister still invited you to the Memorial. Using that as an example of rudeness, how would she feel if you invited her to Christmas knowing how she feels about it? I bet she'd be pretty cheesed off and offended. Instead of getting all up in arms and "how dare you invite me to your Memorial when you know how i feel?!", you simply sent her a scripture that stated your reason for not attending. Basically, you upheld the rules of etiquette by sending her an RSVP along with a reason why you couldn't go lol. It was her choice to be offended at it.
Lastly, remember that in this situation, the only person you can make happy is you. You're not going to make your sister happy unless you fully commit and go back to the JW's...but that would be lying and make you miserable. You're not going to make your mother happy with a continued conflict with your sister. So, the only one left is you. Leave the ball in your sister's court. She knows where to find you if she wants a relationship. The fact that she went out with you says that she does miss her sister, but her cult mindset won't let her miss you enough to completely break the rules. In that case, let her know that you love her and when she wants to be a real sister and not a perfunctory one, you'll be waiting for her with open arms.
